The long range goal of this research is to understand the controls governing the proliferation and differentiation of a multipotent stem cell, the interstitial cell (I-cell) of hydra. The specific aim of this proposal is to develop a library of monoclonal antibodies against cell surface antigens of individual cell types of hydra, in particular the cells of the I-cell system. The library will be used in two ways. 1) The library should enable us to obtain a more complete description of the I-cell system by revealing the extent of the complexity of the I-cell population in terms if sub-populations of I-cell with different functional capabilities. The number of antibodies specific for different sub-set of I-cells, or for sub-sets and one or more differentiation products will be a measure of the complexity. These antibodies will be used to map out the cell lineage relationships among the sub-sets. The proliferation and differentiation capacities of each sub-set will be assayed using an aggregation technique routinely used to study I-cell behavior. 2) The approach to the long range goal is to construct tissues with different cell compositions to determine which cell types influence the I-cells to undergo proliferation and each of five differentiations. The different tissues are formed by making aggregates of cell suspensions of selected cell compositions. Antibodies to individual cell types will be used to work out cell separation procedures. Using isolated cell populations of one or more cell types to increase the precision of selected cell compositions, aggregates will be formed in which the influence of the isolated cell type(s) on I-cell behavior can be determined. An understanding of the cellular interactions controlling the differentiation of multipotent cells is basic to understanding of some classes of defects that occur during human development.